It is known in the art that metallocenes, notably those of Groups 3-6, particularly those including the Group 4 metals of the Periodic Table of the Elements, become active polymerization catalysts in combination with appropriate cocatalyst and are particularly useful in production of polyolefins, particularly when .alpha.-olefins are contacted with such catalyst systems. When the structure of the metallocene is rigidified by a bridging ligand between the aromatic rings, hetero atom, other non-halide or small-group alkyl radical, or combinations thereof, chiral centers arise and it becomes possible to form both rac and meso forms of numerous metallocene-type molecules. With this chirality, the potential for production of tacticity specific polymers becomes possible. This may be accomplished particularly by polymerization of olefins, particularly alpha-olefins, having at least three carbon atoms. Therefore, it is possible to create tacticity differentiated polymers by polymerizing propylene, longer-chain olefins, or combinations thereof.
With bridged metallocenes, it is possible to create both the rac and meso forms of the catalytically active molecule. In polymerization of .alpha.-olefins only the rac isomer is isospecific and therefore able to produce isotactic polymer. To accomplish this it is necessary to isolate the rac isomer or somehow produce only that isomer.
Razavi describes, in U.S. Pat. No. 5,158,920, a process for producing stereospecific polymers. This is accomplished by deprotonation of a cyclopentadiene in a polar solvent to form a solution of the corresponding anionic species followed by steps to create a metallocene from the anionic species.
Winter et al. describe, in U.S. Pat. No. 5,145,819, syntheses of various metallocenes followed by enrichment of the rac isomer with enrichment to levels of more than 17:1 and more than 15:1 rac:meso forms. Two enrichment schemes are reported to obtain these respective results. One involves repetition of a cumbersome scheme of 1) stirring of the racemic metallocene mixture with n-pentane and drying, 2) suspending the resulting solid in THF, and 3) filtering off the suspension. The second involves a toluene extraction of a residue resulting from a late synthesis evaporation step followed by evaporation, followed by THF extraction, which is then followed by recrystallization from toluene.
Kaminsky reports, in U.S. Pat. No. 4,769,510, a synthesis of metallocenes in which temperatures at particular points in the process are carefully controlled, and maintained quite low, to finally attain an enriched mixture with greater than predicted rac form concentration.
Others have worked to reliably produce differentiated tacticity polyolefins and the catalysts enabling such production. The methods described to date have shortcomings of low enrichment, cumbersome multi-step or repetitive processes. We provide means for accomplishing the goal of isolating rac bridged metallocene-type molecules for production of isospecific polymer, particularly poly-.alpha.-olefins. Our method uses a simple, commercially economic repetition of a simple toluene wash to attain enrichment concentrations of rac:meso forms in excess of about 20:1.